The Chinatown International District building where 13 people were slain in Washington state’s deadliest massacre more than 30 years ago now holds another place in the annals of Seattle history: The cause of the Christmas Eve fire that heavily damaged the three-story structure was officially declared undetermined Thursday.

“The building is unsafe,” Fire Chief Gregory Dean announced at a late-afternoon news briefing, where he explained that fire investigators were unable to reach upper parts of the 104-year-old building to carry out the painstaking work of pulling apart evidence and examining burn patterns.

Dean said the “undetermined” designation was permanent.

Eight businesses that operated at street level around the building at 665 S. King St. will “not to be able to open anytime soon,” said Teri Woo, whose family has owned the site for more than 50 years. Woo said she hoped the businesses could eventually reopen.

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Four businesses ­on the west half of the building — a bakery, a fish store, a gospel center and a gift shop — were red-tagged Thursday after the structure was inspected by the Fire Department and the city’s Department of Planning and Development.

Operators may only re-enter for about five minutes with a Fire Department escort to recover valued items.

Yellow tags were affixed to four other businesses, including a restaurant, on the less-damaged east half, allowing the operators to enter with an escort for about an hour to recover belongings.

The two upper floors, which long ago housed a hotel, were vacant. The fire is believed to have started on the top floor, where flames were shooting through the roof when firefighters arrived late Tuesday afternoon.

Timothy Woo, Teri Woo’s brother and the family’s official spokesman, said the family likely faces a “complicated” set of decisions — ranging from repairing the building to demolishing it.

Those options will be guided by the advice of structural engineers the family will be required to hire. He said the family has an insurance policy, but “nothing is sure right now” and further discussion will be needed.

Most pressing, Timothy Woo said, was the need to immediately hire security guards to prevent anyone from entering the building; erect fencing to keep people out of Maynard Alley along the west side; and to install scaffolding to catch debris that could fall from the brick facade.

“Safety is the primary concern right now,” said Woo, who spent much of Thursday standing outside the building waiting for fire and building officials to inform the family of what they had found.

Maynard Alley leads to the vacant basement location of the illegal Wah Mee gambling club where, in the early morning of Feb. 19, 1983, 14 people were hogtied and shot during a robbery. One survived and three men were convicted in connection with the crime.

Police continued Thursday to block off a potential “collapse zone” around the building, preventing cars and pedestrians from traveling directly below.

Traffic will be reopened at some point, officials said, although some parking spaces and sections of sidewalk will remain closed until scaffolding is erected.

Whatever the engineers recommend for the future of the building will have to be reviewed and approved by the city, said Rick Lupton, engineering and codes manager for the city’s Department of Planning and Development.

As Woo family members waited for word on the damage, they spoke of the building’s place in the community.

One sibling, Tanya Woo, referred to the “historical significance” of the building, noting that immigrants who had passed through San Francisco would stop and stay in the hotel on their way to work in the canneries in Alaska.